The estimable Nancy Kress packs a lot of tension into 185 quick pages in After the Fall, Before the Fall, During the Fall, a near-future, apocalyptic time travel thriller that just misses the bullseye because it simply can't resist getting all didactic at the end. It also leaves one key story element deliberately unexplained, which would have been all right had it not been one on which such a large part of the narrative hinges. If at any time in the past, you'd told me that the word "disappointing" could possibly be used in the same sentence as "Nancy Kress novel," I'd have told you you should go have a little lie-down. First time for everything, I suppose.

In 2013, mathematician Julie Kahn is assisting the FBI in solving a baffling series of crimes along the east coast, using software that employs predictive algorithms to anticipate where and when the next ones will occur. Babies and very small children are being abducted. Though there are no connections between the families, some witnesses claim the same thing: the perps are all spindly, odd-looking youths who vanish in a flash of light. In what may or may not be related crimes, a number of retail stores are reporting burglaries in which there is no physical sign of a break-in. Random items are taken while obvious targets like cash registers and safes are left untouched.

We know immediately who's doing all this: a group of youths from just over 20 years in the future. All life on Earth has been wiped out in a series of ecological disasters by the Tesslies, ineffable aliens who are either machines, or lifeforms encased in machines. They have, for reasons that remain obscure, saved a small handful of survivors housed in an enclosed habitat called the Shell. When it became clear that most of the original rescuees had too many birth defects to reproduce successfully, the Tesslies provided the Grab, a device that allows for a quick trip (only seven minutes) into the recent past. Just enough time to snatch a sleeping baby from a crib, or ransack a supermarket or sporting goods store for provisions.

Considering how bleak and grim a lot of young adult fiction has been lately, this is a book, I suppose, that could easily be marketed as YA. Pete, the 15-year-old protagonist from the Shell, is convincingly angst-ridden. Trapped in a life that feels pointless and without much hope, he dreams of any kind of small revenge against the Tesslies. Though approaching manhood, he's still very much a child, and sounds like one when he and a friend make their plans. If Tesslies really are robots, then they'll just find the battery case and pull out the batteries. Problem solving really is that clear to the very young, or the very stupid.

Julie's story is where Kress delivers most of her suspense. While her investigation into the abductions is in a brief lull, a scientist client makes Julie aware of ominous, sudden changes in the ecosystem that haven't hit the press yet. With her methodical mind, she begins adding events up. The brevity of the story, coupled with Kress's efficient storytelling, keeps the tension high.

Too bad that, when it's all said and done, we end up with an on-the-nose lecture about our stewardship of the planet, and how everything that's been going on is all about Gaia punishing us for our evils. It's not a sentiment I entirely disagree with (though I'm loath to anthropomorphize nature, I do think we are on our way to dark days ahead unless we take better care of our only world). Yet it's the sort of thing where people who already agree will find the message trite and patronizing, and the denialist camp will roll their eyes and dismiss the whole affair as tree-hugging propaganda.

But here is where the book really comes up short of complete satisfaction: if the Tesslies are not in fact the ones who set our destruction in motion, then who or what are they? Because they are present, silently monitoring and guiding the actions of the survivors in the Shell. Given Kress's long-proven track record of storytelling excellence, I was expecting some ingenious twist, or at the very least something in the way of a nifty surprise, to address if not entirely resolve the mystery of the Tesslies, to provide the powerful payoff and catharsis we all seek from great storytelling. Instead…well, I won't say. But it was enough of a letdown to make this one of the rare Nancy Kress stories that takes a fall. (Still, it won the 2012 Nebula for best novella, so your mileage may vary.)